1. Field of the Invention
The present invention relates to an electron beam apparatus, such as an electron microscope.
2. Summary of the Prior Art
When an electron microscope is used to investigate a specimen, such as a semiconductor wafer, an electron beam from an electron source is focussed onto the specimen by an objective magnetic lens. When an area of the sample is to be investigated, the electron beam is caused to scan the specimen, either by causing the point of incidence of the electron beam on the specimen to change by control of scanning means controlling the path of the electron beam, or by moving the specimen by moving a mounting stage on which the specimen is mounted. Such scanning occurs in electron beam lithography, and also in critical-dimension measurements, carried out on semiconductor wafers.
However, in such electron beam apparatus, the surface of the sample may not be wholly even, and this is a particular problem with large silicon wafers having diameters of the order of 20 cm. Thus, the height of the surface of the wafer relatively to the objective magnetic lens may change, and this may result in the electron beam becoming out of focus.
For this reason, it has been proposed that means are provided for detecting the change in the wafer height. The focus of the electron beam may then be varied, by controlling the objective lens and/or by moving the mounting stage to move the wafer.
JP-A-63-254649 disclosed such a height measuring means, in which light from a laser was directed onto the specimen at a position corresponding substantially to the point of incidence of the electron beam, and reflection of that light from the specimen was detected by a light detector. If the height of the specimen changed, there would be a change in the point of incidence of light beam on the specimen, and this could be detected by movement of the reflected light beam at the detector. Hence, by controlling the focussing of the electron beam and/or position of the wafer using the information form such a light detector, it was possible to maintain the electron beam in focus.